1. Field of the Invention
This invention is directed to recovering clock pulses of wavelength division multiplexed optical signals and to regeneration of wavelength division multiplexed optical signals. In particular, it relates to simultaneous clock recovery and regeneration of many wavelength division multiplexed optical signals.
2. Technical Background
As the capacity of wavelength division multiplexed (WDM) transmission systems increases in response to the increasing demand for communication, the maximum reach of each transmission system is diminished. Regenerators are therefore required at regular intervals along a transmission link in addition to any regenerators associated with network nodes where traffic routing takes place. It may be argued that regenerators are necessary within switching nodes to provide traffic routing and grooming functions, though this is not always the case when traffic on a given wavelength is routed straight through the node. However, the use of regenerators between nodes increases the network cost without contributing additional functionality. A cost-effective means of regenerating WDM signals is therefore required as an alternative to full WDM demultiplexing and opto-electronic regeneration. System manufacturers indicate that this is particularly necessary for 40 Gbit/s data rate systems with a target reach of 3000 km but a practical transmission limit around 1500 km.
All-optical regenerators which provide for the individual regeneration of each wavelength in a WDM system have been proposed (see, for example, Electronics Letters vol 32 no. 6, pp567, 1996 xe2x80x9cError free operation of a 40 Gbit/s all-optical regeneratorxe2x80x9d by Pender, Widdowson, Ellis; Electronics Letters vol 24 no. 14, pp848, 1998 xe2x80x9cAll-optical regeneratorxe2x80x9d by Giles, Li, Wood, Burrus, Miller). However, such systems require the WDM signals to be demultiplexed, after which each channel is processed by a respective optical regenerator. Such single channel fiber-based regenerators may perform clock recovery using a fiber ring laser mode-locked through cross-phase modulation or non-linear polarization rotation and a decision gate based on similar non-linear properties. The output wavelength is determined by the local pulse source in the ring laser section and is in general substantially different to the incoming wavelength. It is well known that the non-linearity of optical fibers is broadband, and so the device is tunable over a large wavelength range. However, since the device is based on cross-phase modulation or its derivative effect, non-linear polarization rotation, attempts to operate with several wavelengths simultaneously inevitably result in unwanted crosstalk between the channels through the same cross-phase modulation effects.
It has also been suggested that RZ (return to zero) formatted WDM signals may be simultaneously regenerated using soliton transmission and synchronous modulation. However, in this scheme it is necessary to ensure that all WDM signals arrive at the synchronous modulator with the same phase. This offers many practical difficulties arising from different clock sources, different propagation paths and small-scale drift of laser wavelengths coupled with residual dispersion. These difficulties are exaggerated when a wavelength-routed network is contemplated, and the requirements of soliton transmission are taken into account.
Therefore there is a need for an improved method and apparatus for use in all-optical clock recovery and signal regeneration, which can simultaneously process a plurality of WDM signals.
According to a first aspect of the invention, there is provided an apparatus for recovering clock pulses of wavelength division multiplexed optical signals passed therethrough. The apparatus comprises an optically-pumped laser cavity defining a cavity length and comprising a nonlinear medium pumped at a wavelength selected to give efficient parametric amplification within the medium. The cavity length corresponds to an integer multiple of bit periods of at least one of the multiplexed optical signals. The optical signals co propagate through the medium with the pump radiation. The apparatus further comprises an optical path for recirculating a proportion of the output from the laser cavity back through the laser cavity. In this way, idler waves are generated symmetrically about the pump wavelength by four wave mixing with the at least one of the multiplexed optical signals and recirculated through the laser cavity to be amplified by parametric amplification in order to recover wavelength division multiplexed clock pulses.
By pumping the dispersion-shifted nonlinear medium to give efficient parametric amplification, the input data signals interact efficiently with the pump to generate corresponding wavelength converted signals symmetrically spaced either side of the pump. At other non-symmetric wavelengths, group velocity dispersion destroys the phase matching, and results in a weak four wave mixing interaction. Depending on the relative phase of the pump and the symmetrically placed signals, the latter may extract energy from the pump by parametric amplification. Recirculation of the idler waves through the laser cavity enables generation of further idler waves where data pulses may have been absent during previous recirculations, so recovering wavelength-converted clock pulses.
The cavity may be formed between a plurality of reflectors, at least one of which reflectors is a wavelength-selective reflector which is partially reflective at the wavelength of at least one of the idler waves for partially reflecting such idler waves back through the cavity after passage through the cavity. In this way, a proportion of the wavelength converted clock pulse radiation may be recirculated through the cavity with the correct phase, to ensure strong parametric amplification of the idler waves (as described above) and recovery of clock pulses.
Alternatively, the cavity may be formed in a ring laser or a sigma laser configuration. Use of a ring laser configuration overcomes the disadvantages arising from non-linear effects and formation of sub-cavities by reflections inherent in linear cavity configurations.
Preferably, the apparatus further comprises a filter to prevent further transmission of radiation at the pump and signal wavelengths after passage through the nonlinear medium. The filter may comprise a band pass filter within the optical cavity. By blocking the pump and signal wavelengths after passage through the dispersion-shifted medium, one is left with idler waves corresponding to data pulses of the optical signal. Without a means to prevent further transmission of radiation at the pump and signal wavelengths after passage through the nonlinear medium, the recirculated pump and signals would need to be in phase.
Preferably, the apparatus further comprises an adjuster for adjusting the cavity length to correspond to an integer multiple of bit periods of at least one of the multiplexed signals. An adjuster would be required for apparatus using longer cavity lengths where environmental fluctuations are sufficient to induce a measurable change in cavity length. The adjuster may comprise an adjustable fiber delay line. This would provide sufficient phase matching accuracy for a short cavity.
Preferably, the adjustable fiber delay line is actively stabilised to compensate for environmental fluctuations in the cavity.
Preferably, the cavity further comprises dispersion slope compensation. This enables correct cavity length adjustment for a greater range of simultaneous signal wavelengths. Preferably, the dispersion slope compensation has mirror image dispersion characteristics to those of the nonlinear medium, Suitably, the dispersion slope compensation comprises dispersion-compensating fiber. Alternatively, the dispersion slope compensation and band-pass filter comprise at least one fiber grating, and may include a number of gratings per wavelength.
Preferably, the residual dispersion after dispersion compensation is not zero, and the cavity length at each idler wavelength is equal to an integer multiple of bit periods. Preferably, the coherence length of the signals is substantially greater than the cavity length, to ensure correct optical phase matching and maximise phase-sensitive gain.
The filter to prevent further transmission of radiation at the pump and signal wavelengths after passage through the nonlinear medium may comprise an optical branch presenting a series of cascaded chirped fiber Bragg gratings, the optical branch being connected to the optical cavity by an optical circulator, whereby each chirped fiber Bragg grating reflects a different idler wavelength back into the cavity. Each chirped fiber Bragg grating reflects a corresponding recovered channel clock while passing light at all other wavelengths, including the pump, WDM optical signals, amplified spontaneous emissions, four wave mixing terms etc. Furthermore, each grating defines a unique laser cavity length for its reflected wavelength, so that for multi-wavelength operation there is no need to have total cavity dispersion equal to zero.
Preferably, each chirped fiber Bragg grating is at least half as long as the physical spacing of two successive optical pulses in the fiber following at the signal clock rate. In this case, reflection from one end of the grating is delayed with respect to the reflection from the other end of the grating by a full bit period, and phase change of +/xe2x88x92180xc2x0 for the recovered clock signal can be attained by a wavelength shift.
The phase of the pump radiation may be modulated at the frequency of circulation of one of the optical signals through the cavity or an integer multiple thereof up to the bit rate of one of the optical signals, to suppress stimulated Brillouin scattering.
Preferably, the cavity further comprises an optical filter with a free spectral range equal to the clock frequency or a subharmonic of the signal clock frequency to select only one or a limited subset of supermodes, so suppressing supermode noise.
Preferably, the cavity further comprises a weak periodic filter with a free spectral range substantially equal to the wavelength spacing between adjacent channels. In combination with self phase modulation induced spectral broadening, the spectral compression offered by the filter will tend to stabilise the pulse amplitudes of different channels, so reducing the effects of any saturation induced crosstalk which might otherwise arise if two clock pulses are present simultaneously. Alternatively, other forms of amplitude stabilisation may be employed, such as non-linear polarization rotation or 2R regeneration, to reduce the amplitude destabilising effects.
Preferably, the optical cavity comprises polarization-maintaining fiber. The polarization sensitivity of the parametric gain would then ensure that the recovered clock polarization was matched to the incoming signal polarization. Preferably, the axes of the fiber are swapped at regular intervals to reduce walkoff.
Preferably, the pump wavelength lies between two adjacent standard channel wavelengths. More preferably, the pump wavelength lies mid-way between the two adjacent wavelengths. The input signals may correspond to even-numbered standard channel wavelengths with the recovered clock channels corresponding to odd-numbered standard channel wavelengths or vice versa, and the input signals may be separated from recovered clock signals using optical interleavers.
Alternatively, the signals may be located within a band of width N, which band is offset from the pump wavelength by a spacing of at least 2N.
The nonlinear medium may comprise dispersion-shifted fiber, a semiconductor optical amplifier or a periodically-poled lithium niobate (PPLN).
In a second aspect, the invention provides a multi-wavelength optical regenerator for recovering and re-timing wavelength division multiplexed optical signals, the regenerator comprising apparatus for recovering clock pulses of WDM optical signals as above, wherein wavelength division multiplexed optical signals and the recovered clock pulses are coupled to a further optically-pumped amplifier comprising a nonlinear medium pumped at a wavelength selected to give efficient parametric amplification within the medium, whereby the optical signals and the recovered clock pulses are modulated by each other.
Preferably, the regenerator comprises a filter to prevent further transmission of radiation at the pump and signal wavelengths after passage through the cavity and further amplifier, leaving only the modulated recovered clock signal. Alternatively, the regenerator comprises a filter to prevent further transmission of radiation at the pump and recovered clock wavelengths after passage through the cavity and further amplifier, leaving the original optical data signal modulated by the recovered clock.
According to a third aspect of the invention, there is provided a method for recovering clock pulses of wavelength division multiplexed optical signals. This method comprises the steps of providing a laser cavity defining a cavity length and comprising a nonlinear medium, optically-pumping the laser cavity at a wavelength selected to give efficient parametric amplification within the medium, and copropagating the optical signals through the nonlinear medium with the pump radiation. The method further comprises the steps of recirculating a proportion of the output from the laser cavity back through the laser cavity, and adjusting the cavity length to correspond to an integer multiple of bit periods of at least one of the multiplexed optical signals. In this way, idler waves are generated symmetrically about the pump wavelength by four wave mixing with the at least one of the multiplexed optical signals and amplified by parametric amplification in order to recover wavelength division multiplexed clock pulses.
The idler waves may be partially reflected back through the cavity.
Preferably the optical signal is subject to excess self phase modulation within the cavity. In this way, each recovered clock may slightly adjust its operating wavelength and hence (through residual cavity or grating dispersion) the effective cavity length, and so relative phase, to ensure maximum gain.
Suitably, the method further comprises compensating signal chromatic dispersion within the cavity.
Preferably the net small signal gain through the cavity is less than one. In this way, any signals generated by spontaneous processes within the cavity and through parametric amplification of incoming amplified spontaneous emissions, will be reduced with each re-circulation through the cavity. Preferably, the net large signal gain will be greater than or equal to one. In this regard, large signals would comprise the recovered clock signals and small signals would comprise any signals having half the intensity or less than the large signals.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiments of the invention, and together with the description serve to explain the priciples and operation of the invention.